Determining measuring uncertainty or error of a PDL-tester

ABSTRACT

Determining a measuring uncertainty and/or maximum measuring error of a polarization dependent loss—PDL—tester for determining a PDL value of a device under test—DUT—is provided by using the PDL tester for determining a value of PDL of a verification element having an actual value of PDL greater than a maximum value of a specified measuring range, wherein the PDL tester has an expected measuring uncertainty and/or expected maximum measuring error. The measuring uncertainty and/or maximum measuring error or the tester is then derived from the determined value of PDL of the verification element in conjunction with the actual value of PDL of the verification element.

This application is the National Stage of International Application No.PCT/EP02/07869, International Filing Date, Jul. 16, 2002, whichdesignated the United States of America, and which internationalapplication was published under PCT Article 21(2) as WO Publication No.WO 03/078956 A1 and which claims priority from European Application No.EP 02004701.5, filed Feb. 28, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to determining a measurinq uncertainty orerror of a tester for polarization dependent loss (PDL).

2. Brief Description of Related Developments

PDL is usually understood as the difference between maximum and minimumloss (normally in dB) of a device under test (DUT) when changing thepolarization of a light beam sent through the DUT. PDL testers aretesting or measuring devices adapted to measure or determine the PDL ofthe DUT. The specification of the PDL tester represents the measurementuncertainty of the tester for a given PDL of the DUT. To verify thespecification of the PDL tester, normally a reference device (so-calledgolden device) with known value of PDL is used.

SUMMARY OF THE INVENTION

It is an object of the invention to provide improved determining ofmeasuring uncertainty or error of a tester for PDL. The object is solvedby the independent claims. Preferred embodiments are shown by thedependent claims.

For the sake of avoiding repetitions, the term ‘PDL’ as used in thefollowing is usually to be understood as a value of PDL, if nototherwise clear from the context. Accordingly, the term tester isusually to be understood as PDL tester.

An advantage of preferred embodiments of the present invention is thepossibility to avoid the necessity to use costly reference deviceshaving a precisely defined and precisely known PDL as verificationelements, so called “golden devices”. Therefore, the small but existinguncertainty of such reference devices worsening the system specificationof the tester can be avoided.

Preferred embodiments of the invention allow using standard connectorsto connect the verification element e.g. to a seat of the tester. Suchstandard connectors have the disadvantage in the prior art that theyshow an amount of PDL, which gives additional uncertainty to the systemspecification of the tester. Thus, e.g. expensive and time consumingsplicing of the fiber at the connecting sites of the tester to avoid thePDL of connectors can be omitted following the preferred embodiments.

In preferred embodiments of the invention, simply the PDL measurement ona verification element with high PDL, here also called polarizer, withthe tester is sufficient to provide measurement results which can beused to evaluate an upper limit for the error or measurement uncertaintyof the tester for a given DUT having a given amount of PDL.

In case the PDL of the verification element is sufficiently high withrespect to the measuring range of the PDL tester, the actual PDL of theverification element needs not be precisely known and can be assumed tobe infinite. It is sufficient if the actual PDL of the verificationelement is much higher than the maximum PDL of the specified measuringrange of the tester. Preferably, the actual PDL of the verificationelement is selected to be higher than a maximum meaningful or specifiedvalue of the PDL to be measured by the tester. It is preferred to useverification elements having at least 10 dB, preferred at least 20 dB,more preferred at least 30 dB of PDL. Because of the high PDL of theinventive verification element, the PDL e.g. of the connectors play norole any more.

The error or measurement uncertainty of the tester might depend on theorientation of the DUT (i.e. on the polarization vectors of the DUT formaximum and minimum transmission). This can be covered by repeatedlymeasuring with the tester the polarizer under different orientationsand/or with different positions and loops of the polarizer patch cordand/or with different polarizers (e.g. with polarizers with differentvectors for maximum/minimum transmission), so that the polarizerorientation changes with respect to the polarization of the source. Theworst case measured polarizer PDL, e.g. the lowest PDL result, can thenbe taken for evaluation of the estimated worst possible testermeasurement error.

In one embodiment, an approximation of the error or measurementuncertainty of the tester is achieved, as described in more detailbelow, by using the formula:

$\begin{matrix}{E_{nom} \propto {{PDL}_{nom} - {10 \cdot {\log\left( \frac{10^{{PDL}_{nom}/10} + 10^{{- {PDL}_{{pol},m}}/10}}{1 + 10^{{({{PDL}_{nom} - {PDL}_{{pol},m}})}/10}} \right)}}}} & {{eq}.\mspace{11mu} 1}\end{matrix}$for the evaluation of the maximum possible error of the tester, withPDL_(nom) being the nominal value of the PDL in dB of a device undertest (DUT) to be measured, PDL_(pol,m) being the PDL in dB of theverification element measured by the tester (the minimum value in caseof repeated measurements) and E_(nom) being the upper limit for themaximum possible measurement error in dB of the tester at the nominalPDL of the DUT.

In another embodiment the following simplified formula:

$\begin{matrix}{E_{nom} \propto {10 \cdot {\log\left( \frac{10^{{PDL}_{nom}/10} + 10^{{PDL}_{{pol},m}/10}}{10^{{- {PDL}_{nom}}/10} + 10^{{PDL}_{{pol},m}/10}} \right)}}} & {{eq}.\mspace{11mu} 2}\end{matrix}$using the same variables is applied instead of equation 1.

The invention can give lower errors for the test results of the testerwithout changing the tester, because the verification is performed moreprecisely with the help of the invention since connector PDL and changesof polarizer PDL do less or not contribute to the test uncertainty.

In a preferred embodiment of the invention there is provided a zerodevice (e.g., a simple standard single mode fiber patch cord) to measurethe intrinsic PDL of the tester or deficiencies of the tester that actlike intrinsic PDL, e.g. power meter noise, light source powerfluctuation, connector PDL, power meter PDL etc. The total uncertaintyof the tester is composed by the uncertainty derived from the polarizertest and from the intrinsic PDL of the tester.

Finally, it is possible in preferred embodiments to make theverification of the tester in transmission and in reflection.Preferably, the verification with the polarizer as the inventiveverification element is done only in transmission since the results canbe used for the evaluation of the specification of reflection, also.Alternatively or additionally the verification can be performed inreflection using a reflective verification element with high PDL(reflective polarizer). However, both in transmission and in reflectionthe measurement with the zero device is preferred. The transmission zerodevice can simply be a patch cord, the reflection zero device can be areflector.

It is clear that the invention can be partly embodied or supported byone or more suitable software programs, which can be stored on orotherwise provided by any kind of data carrier, and which might beexecuted in or by any suitable data processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated and become better understood byreference to the following detailed description when considering inconnection with the accompanied drawings. The components in the drawingsare not necessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Features that aresubstantially or functionally equal or similar will be referred to withthe same reference sign(s).

FIG. 1 shows a schematic illustration of an embodiment of the invention,

FIGS. 2, 3 show graphic illustrations of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an embodiment 1 shows a light source 2 which provides apolarized light beam 3 to a polarization controller 4 which providese.g. four polarization states to the beam 3 (e.g. for the Mueller-Stokesmethod) or multiple polarization states (e.g. for the polarizationscanning method, where the maximum/minimum loss is searched bycontinuously or step-like changing the polarization). The accordinglypolarized beam 3 is then provided to a verification element 6 in a (notshown) seat of apparatus 1 for PDL measurements.

The verification element 6 having a polarizing feature replaces a DUT.The polarizing feature of the verification element 6 is a relativelyhigh PDL compared to the PDL of the DUT to be measured with setup 1.E.g. the verification element 6 used in setup 1 has a PDL which is tentimes higher than the PDL to be measured with the tester 1, i.e. in theshown embodiment the PDL of the verification element 6 is at least 30dB. It is important to notice that the PDL of the verification element 6needs not be known exactly. It is sufficient to have a minimum actualPDL of the verification element 6 over all relevant parameters liketime, temperature, wavelength. Therefore, it is sufficient for the shownsetup that it is known that the verification element 6 of the presentembodiment has a minimum PDL of 30 dB. It is also possible to use averification element with a lower actual PDL, e.g. with a PDL of 18 dB,but this can result in more pessimistic (=higher) estimations of thetester PDL measurement uncertainty.

Having a verification element 6 with such a great PDL it is easilypossible to make the contact between the verification element 6 and thetester with normal connectors (not shown) since the additional PDLuncertainty of such plugs in a range of approximately 20 mdB can beneglected. Therefore, expensive splicing of the “not shown” fiber forbeam 3 is not necessary. Also because the PDL of the verificationelement 6 does not have to been known precisely, no aging or temperatureproblems arise. Finally, the verification element 6 according to thepresent invention is low cost (typically less then one tenth) withrespect to a so-called “golden” device as a reference device used in theprior art.

A preferred embodiment of the inventive method works as follows:

In this embodiment a zero measurement is performed first of all. By thisintrinsic PDL effects are covered, e.g. noise of the power meter 8,light source power fluctuations, PDL of internal connectors of thetester, power meter PDL. The zero measurement is performed e.g. byintroducing a (not shown) patch cord into the seat of the tester insteadof the verification element 6. Alternatively, it is possible to performthe zero measurement by just connecting two connectors of the seatdirectly with each other.

Having performed the zero measurement a PDL measurement is performedwith the verification element 6 in the seat of the tester by a polarizermeasurement. This polarizer measurement gives information aboutimperfection of the different polarization states, e.g. polarizationcontroller errors, quasi-unpolarized background from source spontaneousemission (SSE) of a laser and further unknown contributions, since thesecontributions scale with the PDL of a DUT to be measured with tester 1.

Having measured the PDL of the verification element 6, the maximalpossible error of the tester 1 is derived by using the aforementionedequation 1 for the evaluation of the maximum possible error of thetester 1, with PDL_(nom) being the nominal value of the PDL in dB of theDUT, PDL_(pol,m) being the PDL in dB of the verification element 6measured by the tester 1 (the minimum value in case of repeatedmeasurements), and E_(nom) being the upper limit for the maximumpossible measurement error in dB of the tester 1 at the nominal PDL ofthe DUT. Instead, the simplified formula of equation 2 can be usedaccordingly.

Alternatively or additionally the formula

$\begin{matrix}{U_{rel} \propto {1 - \frac{10^{{PDL}_{{pol},m}/10} - 1}{10^{{PDL}_{{pol},m}/10} + 1}}} & {{eq}.\mspace{11mu} 3}\end{matrix}$can be used for the evaluation of the maximum possible relativemeasurement error of the tester with respect to the PDL of a DUT to bemeasured in dB, PDL_(pol,m) being the PDL in dB of the verificationelement measured by the tester (e.g. the minimum value in case ofrepeated measurements). This is at least valid in the linear regime asdescribed later in description of FIG. 3. U_(rel) might be “2%” forexample.

The formulas are based on the Mueller-Stokes method as described e.g. in“Dennis Derickson, Fiber Optic, Test and Measurement, Prentice-Hall,Inc., Upper Saddle River, N.J. 07458, USA, 1998, pages 232–234 and356–358”and the following described error model:

The Mueller-Stokes method for PDL measurement is based on lossmeasurement at four defined polarization states of the incident light 3that are orthogonal on the Poincaré-Sphere. The measurement result erroror uncertainty depends on the linearity of the power meter 8 (inclusivenoise) and the perfection of the polarization states (inclusive powerstability of the source 2). Additional uncertainties are known to resultfrom the PDL of the systems components (e.g. connectors, coupler, powermeter 8 etc.). From the four loss results the polarization dependentloss of the verification element 6 can be evaluated. The four differentpolarization states are introduced in the system by-the polarizationcontroller 4 which provides four different polarization states SOP 1,SOP 2, SOP 3, SOP 4 e.g. horizontal polarization, vertical polarization,diagonal polarization, circular polarization.

Deviation from orthogonality results in a PDL measurement erroraccording to the following explanation: when comparing two nominalorthogonal polarization states (“1” and “2”), e.g. like “horizontal” and“vertical”, state “2” can be separated in a part orthogonal to “1” andone part parallel to it. A loss measurement in state “2” thereforeresults in a resulting loss that is composed by a part orthogonal to “1”and a part parallel to “1”. The latter leads to PDL measurement error.The same is valid for example for an unpolarized contribution.

The PDL measurement uncertainty can be estimated by measuring a perfectpolarizer (PDL=∞). If the polarizer is in such position that state “1”is maximum transmission, the transmission in state “2” would be ideallyzero, the actually measured transmission gives a measure for the PDLmeasurement error.

E.g. by “moving” the cable between polarization controller 4 andpolarizer (generally: by making an orthogonal transformation of thepolarization state at the polarizer input), and performing a new PDLmeasurement, the worst case measurement error for DUTs of differentorientation of the polarization eigen vectors can be found. The eigenvectors are the states of polarization with maximum or minimum loss.

For example the worst case (i.e. lowest) PDL result measured by thesystem under test for the used verification element or polarizer can beused for evaluation (other possibilities: a statistical worst caseresult calculated from a series of measurements is used. Furthervariation: not the PDL result is used for evaluation, but the derivedratio of maximum and minimum transmission of the polarizer (which isrelated to the PDL value)).

Furthermore, the tester 1 can be designed to measure PDL in reflection,also. For this purpose a (not shown) coupler is installed betweenpolarization controller 4 and the seat for the verification element 6.Then the same PDL measurements can be done as in transmission, either byscanning with a motorized polarization scrambler (i.e. an apparatus ableto modify the polarization of the transmitted beam) or by providing fourdifferent polarization states with a polarization controller 4 forMueller-Stokes analysis. Connected to the coupler is an additionalsecond (not shown) power meter for measuring the loss values of theverification element 6 for the given polarizations in reflection.However, it is not necessary to perform the measurement of theverification element 6 in reflection. Instead it is only necessary tohave a zero measurement in reflection. This is because the way betweenthe light source 2 and the seat for the DUT is the same in transmissionand in reflection, so that imperfections of the polarization states havethe same effect in transmission and in reflection. Thus the results ofthe polarizer measurements in transmission can be used for reflection,also.

To perform the zero measurement in reflection there is used a reflectorpositioned in the seat instead of the verification element 6. Theintrinsic PDL derived from the zero measurement increases themeasurement uncertainty of the tester. Both contributions are added in ssuitable way (e.g. arithmetically or by root-sum-square).

FIG. 2 shows a graph displaying the minimum measured polarizer PDL“PDL_(pol,m)” on the x axis and the calculated tester measurementuncertainty “E_(nom)” on the y axis with a nominal PDL “PDL_(nom)” of aDUT to be tested assumed to be 0.5 dB as example. The curve shows thatthe uncertainty “E_(nom)” is getting smaller when the measured PDL“PDL_(pol,m)” of the verification element 6 is enlarged. As a minimumvalue of the actual PDL “PDL_(pol,m)” of the verification element 6 itshould be chosen a value which is greater than the targeted PDL“PDL_(nom)” of the DUT to be measured. Preferably, it should be chosen avalue that corresponds (according to the given formulas and FIG. 2) to aresulting uncertainty “E_(nom)” that is smaller than the uncertainty forwhich the tester is designed for. In embodiment 1 the PDL “PDL_(pol,m)”of the verification element 6 is chosen to be 30 dB corresponding to theuncertainty +0.001 dB being smaller than the targeted tester PDLuncertainty of e.g. ±0.01 dB.

FIG. 3 shows a graph showing nominal PDL “PDL_(nom)” of a DUT to betested on the x axis and the calculated expected tester uncertainty“E_(nom)” for a system with minimum measured-polarizer PDL “PDL_(pol,m)”of the verification element 6 being 20 dB. The upper solid line showsthe uncertainty “E_(nom)” calculated according to the above-mentionedformula. The lower broken line shows an asymptote of the calculateduncertainty “E_(nom)” being a linear approximation of the upper curve.It can be seen that the linear approximation has a slope of 2%.Therefore, for example, the relative uncertainty of 2% can be used asthe specification of the tester 1 when the nominal PDL “PDL_(nom)” ofthe DUTs to be tested is in the “linear regime”, here between 0 and 2dB.

1. A method of determining a measuring uncertainty maximum measuringerror of a polarization dependent loss—PDL—tester adapted fordetermining a PDL value of a device under test—DUT—, comprising thesteps of: (a) using the PDL tester for determining a value of PDL of averification element having an actual value of PDL greater than amaximum value of a specified measuring range, wherein the PDL tester hasan expected measuring uncertainty or expected maximum measuring error,and (b) deriving the measuring uncertainty or maximum measuring error ofthe tester from the determined value of PDL of the verification elementin conjunction with the actual value of PDL of the verification element.2. The method of claim 1, wherein the actual value of PDL of theverification element is at least one of the following: at least tentimes greater than the maximum value of the specified measuring range ofthe PDL tester, or at least 10 dB.
 3. The method of claim 1, whereinstep b comprises a step of approximating the measuring uncertainty ormaximum measuring error of the tester by at least one of: using thedetermined value of PDL of the verification element in conjunction withthe actual value of PDL of the verification element, and using thedetermined value of PDL of the verification element and assuming theactual value of PDL of the verification element to be infinite.
 4. Themethod of claim 1, further comprising the steps of: repeating thedetermination step a at least one time with at least one of thefollowing measures: using a different orientation of the verificationelement, using a different verification element, using a differentposition of at least one device or component in a transmission path of alight beam provided to the verification element for determining itsvalue of PDL, performing a polarization transformation before theverification element, using at least one of a polarization scrambler, afaraday rotator, and a retarder plate provided in the transmission pathbefore the verification element; determining a lowest value of thedetermined values of PDL in step aby applying a statistic determination,and using the determined lowest value of PDL as the determined value ofPDL of the verification element in step b.
 5. The method of claim 1,wherein in step a, the value of PDL of the verification element isdetermined in at least one of transmission and reflection.
 6. The methodof claim 1, further comprising the steps of: (c) using the PDL testerfor determining a zero value of PDL of a zero element having an actualvalue of PDL approximately zero, and (d) deriving a zero measuringuncertainty or zero measuring error of the tester from the determinedzero value of PDL of the zero element.
 7. The method of claim 6, furthercomprising the steps of: repeating the determination step c at least onetime with at least one of the following measures: using a differentorientation of the verification element, using a different zero element,using a different position of at least one device or component in atransmission path of a light beam provided to the zero element fordetermining its value of PDL, performing a polarization transformationbefore the zero element using at least one of a polarization scrambler,a faraday rotator, and a retarder plate provided in the transmissionpath before the zero element; determining a highest value of thedetermined values of PDL in step c by applying a statisticdetermination, and using the determined highest value of PDL as thedetermined value of PDL of the zero element in step d.
 8. The method ofclaim 6, further comprising a step of deriving a total measuringuncertainty or total maximum measuring error of the tester from themeasuring uncertainty or maximum measuring error of the tester asderived in step a in conjunction with the zero measuring uncertainty orzero measuring error of the tester as derived in step c.
 9. The methodof claim 1, wherein the step b comprises a step of deriving a value ofE_(nom), as the measuring uncertainty or maximum measuring error of thetester at a value PDL_(nom) as the PDL of a device under test—DUT—to bemeasured by the tester, using the following formula:${E_{nom} \propto {{PDL}_{nom} - {10 \cdot {\log\left( \frac{10^{{PDL}_{nom}/10} + 10^{{- {PDL}_{{pol},m}}/10}}{1 + 10^{{({{PDL}_{nom} - {PDL}_{{pol},m}})}/10}} \right)}}}},$with PDL_(pol,m) being the value of PDL of the verification element asdetermined in step a.
 10. The method of claim 1, wherein the step bcomprises a step of deriving a value of U_(rel), as a relative measuringuncertainty or relative measuring error of the tester at a valuePDL_(nom) as the PDL of a device under test—DUT—to be measured by thetester, using the following:${U_{rel} \propto {1 - \frac{10^{{PDL}_{{pol},m}/10} - 1}{10^{{PDL}_{{pol},m}/10} + 1}}},$with PDL_(pol,m) being the value of PDL of the verification element asdetermined in step a.
 11. A software program or product stored on acomputer readable medium, for executing the method of claim 1, when runon a data processing system such as a computer.
 12. An apparatus adaptedfor determining a measuring uncertainty or maximum measuring error of aPDL tester that is adapted for determining a PDL value of a device undertest—DUT—, comprising: a receiver adapted for receiving from the PDLtester a value of PDL of a verification element having an actual valueof PDL greater than a maximum value of a specified measuring range,wherein the PDL tester has an expected measuring uncertainty or expectedmaximum measuring error, and an evaluating unit adapted for deriving themeasuring uncertainty or maximum measuring error of the tester from thedetermined value of PDL of the verification element in conjunction withthe actual value of PDL of the verification element.
 13. An apparatusfor determining a measuring uncertainty or maximum measuring error of aPDL tester adapted for determining a PDL value of a device undertest—DUT—, comprising: a verification element having an actual value ofPDL greater than a maximum value of a specified measuring range, whereinthe PDL tester has an expected measuring uncertainty or expected maximummeasuring error, and an evaluating unit adapted for determining a valueof PDL of the verification element and for deriving the measuringuncertainty or maximum measuring error of the tester from the determinedvalue of PDL of the verification element in conjunction with the actualvalue of PDL of the verification element.